cmd/{containerboot,k8s-operator}: allow users to define tailnet egress target by FQDN (#10360)

* cmd/containerboot: proxy traffic to tailnet target defined by FQDN

Add a new Service annotation tailscale.com/tailnet-fqdn that
users can use to specify a tailnet target for which
an egress proxy should be deployed in the cluster.

Updates tailscale/tailscale#10280

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/k8s-operator/operator.go

@@ -10,6 +10,7 @@ package main
 import (
 	"context"
 	"os"
+	"regexp"
 	"strings"
 	"time"
 
@@ -322,3 +323,10 @@ func serviceHandler(_ context.Context, o client.Object) []reconcile.Request {
 	}
 
 }
+
+// isMagicDNSName reports whether name is a full tailnet node FQDN (with or
+// without final dot).
+func isMagicDNSName(name string) bool {
+	validMagicDNSName := regexp.MustCompile(`^[a-zA-Z0-9-]+\.[a-zA-Z0-9-]+\.ts\.net\.?$`)
+	return validMagicDNSName.MatchString(name)
+}